The present invention relates to an electrode assembly for a rechargeable electrochemical cell having a nickel positive electrode subject to swelling on recharging, and a pasted negative electrode, and in one aspect to an electrode assembly of the foregoing type wherein a portion of an exterior face of a substrate of that portion of an electrode plate defining the outermost or peripheral layer or wrap of the electrode assembly is substantially exposed and contacts at least a portion of a container as assembled in a sealed electrochemical cell.
Conventionally, electrode assemblies for electrochemical cells are formed from two separate electrode plates of opposite polarity with a layer of interposed separator material. The negative electrode plate can be of a pressed or pasted design. An aqueous mixture of an electrochemically active material and a binder may be applied to each face of an electrically conductive, perforated substrate and pressed on to the substrate, for example by passing the substrate between rollers. The substrate can be stippled to improve adhesion between the substrate and the electrochemically active material.
The positive nickel electrode may be of a sintered design. A perforated or wire mesh nickel or nickel-plated steel substrate of, for example, 2-3 mils thickness, is sintered with a carbonyl nickel powder layer or layers to form a porous electrode plaque of, for example, 20-30 mils thickness. The resultant porous plaque is conventionally impregnated with a solution of an electrochemically active material precursor, typically nickel nitrate. The electrochemically active nickel hydroxide material is precipitated out of solution within the plate.
In addition, an ultra high porosity nickel positive electrode plate can be utilized in which an electrochemically active nickel material may be bonded to a porous substrate such as a highly porous metal foam or fibrous mat by, for example, pressing a slurry or paste containing the active material on to and within interstices of the substrate. The substrate may then be compacted to form a positive nickel electrode plate having a desired thickness.
In sealed rechargeable nickel electrode-containing cells, such as commercial sealed nickel-cadmium cells employing sintered nickel electrodes, the nickel electrode increases in thickness during cycling. Thickening is believed to be related to the ratio of gamma nickel hydroxide present to beta III nickel hydroxide, the gamma form occupying more space. Such swelling tends to significantly shorten cell life, oftentimes because of premature shorting of the electrode plates. Various efforts have been made in the past to eliminate or inhibit this swelling phenomenon. Only limited success has been realized.
Accordingly, it is an object of this invention to produce a rechargeable nickel electrode-containing electrochemical cell in which means are incorporated to retard the normal tendency of the nickel electrode to swell during the life of the cell.
It is a further object of the present invention to provide a wound or nonwound electrode assembly for use in a sealed nickel electrode-containing electrochemical cell having means retarding swelling of the nickel electrode and which provides for direct electrical contact between a portion of the substrate of the negative electrode plate thereof and a portion of the cell container.
Another object of the present invention is to provide a wound electrode assembly which can be easily manufactured and as assembled within a sealed nickel electrode-containing electrochemical cell will result in a substantially higher coulombic cell capacity and improved cell performance characteristics including extended life due to retardation of short circuiting.